Drive apparatus for print rollers for printing machinery

ABSTRACT

Means for controlling a shunt wound motor for driving the printing roller of a printing machine which includes electrical circuits for controlling the speed of rotation of the motor and for applying a braking force to the motor at the end of the printing operation, which force is proportional to the speed of the motor.

United States Patent Robinson [4 May 23, 1972 [54] DRIVE APPARATUS FOR PRINT [56] References Cited ROLLERS FOR PRIN I'ING MACHINE RY UNITED STATES PATENTS [72] Imam" 22m Readmg Bekshlre' 2,760,135 8/1956 Hillyer ..3 18/368 x 2,629,849 2/1953 Barnes ..3 1 8/261 [73] Assignee: Antoma (Hairdressing) Company Limited Primary ExaminerBernard A. Gilheany [22] Filed 1969 Assistant ExaminerW. E. Duncanson, Jr. [21] Appl. No.: 852,329 AttorneyStevens, Davis, Miller&Mosher [57] ABSTRACT [30] Foreign Application Priority Data Means for controlling a shunt wound motor for driving the Aug. 28, 1968 Great Britain ..4l,l69/68 printing roller of a printing machine which includes electrical circuits for controlling the speed of rotation of the motor and [52] 1.5. CI ...,3l8/26l for applying a braking force to the motor at the end f the [51] Int. Cl. H02p 3/14 printing operation, which force is proportional to the speed of [58] Field of Search ..3 18/258, 259, 261, 269, 270, the moton 3 Claims, 8 Drawing Figures Patented May 23, 1972 6 Sheets-Sheet 1 Patented May 23, 1972 6 Sheets-Sheet FIG. IB

Patented May 23, 1972 3,665,275

6 Sheets-Sheet 5 Patented May 23, 1972 3,665,275

' 6 Sheets-Sheet 4 Patented May 23, 1972 3,665,275

6 Sheets-Sheet 5 Patented May 23,1972

6 Sheets-Sheet 6 DRIVE APPARATUS FOR PRINT ROLLERS FOR PRINTING MACHINERY The present invention relates to drive apparatus for print rollers for printing machinery. The invention is particularly applicable to proof-printing machinery, although there is no limitation in this respect.

In conventional proof-printing machinery it is necessary that the correct register is obtained when proof-printing. In particular, when proof-printing for color prints on a flat bed proof-printing machine, the print roller must occupy the same series of positions to a high degree of accuracy. Conventionally the position of the print roller is controlled by ensuring that the roller starts its movement from a position relative to the flat bed which is identical in every printing operation, and the roller is advanced from this start position by suitable means which ensure that the roller rolls forward directly and does not slip or slide. For example, a rack and gearwheel or a chain and gear wheel are used.

To ensure that the roller starts in the correct position, it is clearly advantageous for the print roller to terminate the return stroke from its previous printing movement in the required correct position. One known method for terminating the return stroke of the roller is by mechanical means which are heavy, expensive to produce, and liable to rapid wear particularly by jarring the print roller by too rapid deceleration. Other known methods employ magnetic clutch arrangements or braking motors, all of which are heavy and expensive to produce. Another known method utilizes electronic means which detects the speed and position of the print roller during its return stroke and feeds back information to braking means for slowing down the roller, the electronic means being very expensive to produce.

An aim of this invention is to control the movement of a print roller by a drive apparatus which is accurate to use and is cheap to produce compared with known drive apparatus.

According to the present invention, there is provided drive apparatus for a print roller for printing machinery, including an electric motor for driving the print roller, and an electrical control circuit for controlling rotation of the motor and for controlling braking of the motor proportionate to the speed of rotation of the motor. Preferably, for braking the motor, the electrical control circuit includes means for switching the circuit to cause the motor to act as a generator.

The control circuit may include means for actuating the circuit to rotate the motor in a forward direction, and means for maintaining the flow of an electric current through either the field windings or the armature windings of the motor when the supply of electric current to the armature windings or the field windings of the motor respectively is stopped.

Preferably, the control circuit includes a first relay for actuating the circuit to rotate the motor in a forward direction, a second relay for actuating the circuit to rotate the motor in a reverse direction, a third relay controllable by the first and second relays individually for actuating the circuit to cause electric current to flow in the field windings of the motor, and a delay circuit for temporarily maintaining the third relay in its actuating state when the respective one of the first or second relays has ceased to actuate.

The apparatus may include a carriage in which the print roller is rotatably mounted, a bed on which the carriage is mounted for reciprocation and means for holding the carriage in a predetermined start position relatively to the bed such that the print roller is in a predetermined position relative to both the carriage and the bed. The holding means may include a buffer spring acting between the carriage and the bed, and

.may include a latch, and a spring urging the latch such that, in

the start position of the carriage, the latch spring causes a reaction from a face of the latch which opposes the force due to the buffer spring, the said reaction and said force being in stable equilibrium.

An embodiment of the invention will now be described by way of example with reference to the accompanying schematic drawings in which:

FIGS. 1A and 13, when joined together along the common line A-A, show a circuit diagram for a drive motor of a flat bed proof-printing machine;

FIG. 2 shows a circuit diagram of an alternative supply transformer for the circuit shown in FIG. 1;

FIG. 3 shows a perspective view of a flat bed proof-printing machine; and

FIGS. 4, 5, 6 and 7 show details of the machine of FIG. 3.

The flat bed proof-printing machine shown in FIGS. 3 to 7 includes a base 41 supporting a plate bed 42, and a print roller carriage 43 mounted above the bed 42. The carriage 43 sup ports a print roller 44, two inking rollers 45, and an ink spreading roller 46. The ink spreading roller 46 is pivotably mounted on the carriage 43 by two arms 47 such that the roller 46 can be pivoted away from the inking rollers 45 for access thereto.

The print roller 44 has a set of sheet-holding clips 48 for fastening a proof sheet to the roller 44. A sheet feed table 49 is mounted on the base 41 at the rear of the machine.

The carriage 43 is movable across the plate bed 42 by means of an electric motor (not shown) mounted on the far side of the carriage 43, as seen in FIG. 3, and arranged to rotate the print roller 44. Gearwheels 51 mounted at opposite ends of the print roller 44 engage in corresponding racks 52 mounted on opposite sides of the bed 42. A second pair of racks 53 are mounted by the side of the racks 52 for rotating one of the inking rollers 45 by means of gearwheels mounted at opposite ends of the inking roller 45.

The drive of the carriage 43 in the forward direction is terminated by a forward stop microswitch S4 and the corresponding reverse drive is stopped by a reverse stop microswitch 55. The microswitches 54 and 55 are operated by an actuator plate 50 (see FIG. 6) rigidly mounted on the side of the carriage 43 by an arm, the actuator plate and arm not being shown in FIG. 3 for the sake of clarity of the figure.

A safety gate 56 (see also FIG. 7) is mounted at the forward end of the carriage 43. The gate 56 is pivotably mounted at its end in the side walls of the carriage 43 and a cam 57 is rigidly mounted to each of the ends of the gate 56 such that, when the gate 56 is in its vertical position, as shown in FIG. 3 the roller of a safety microswitch 58 engages in a notch in the cam 57 and, when the safety gate 56 is rotated in either direction as shown in FIG. 7, the microswitch 58 is operated by its roller being displaced out of the notch in the cam 57.

The print roller 44 is mounted in bearings, which in turn are mounted in eccentric bearings, and is displaceable relatively to the carriage 43 in the downward direction for proof-printing by means of gearwheels mounted at opposite ends of the roller 44, on the eccentric bearings. Each of these gearwheels is rotatable by a short vertically arranged, rack which in turn is displaceable in the vertical direction by an actuator arm 60 terminating in a roller 61 at the lower most end of the arm 60. The roller 61 is arranged to engage with two cam switches 62 and 63 mounted on the side of the base 41 (see also FIG. 5). Each cam switch has a cam lever spring urged into the position shown in FIG. 5. The first cam switch is arranged to displace the arm 60 upwards for rotating the eccentric bearing past a top dead center position so that the print roller is displaced downwards onto the plate bed 42, and the second cam switch 63 is arranged to displace the actuator arm 60 in the downward direction and thereby lift the print roller 44 off the bed 42.

In FIG. 3 the carriage 43 is shown in its start position in which it is held by mechanical means shown in greater detail in FIG. 4. The mechanical means include a buffer spring 65 mounted on each side of the base 41 and a latch 66 co-operating with the buffer spring 65. The latch 66 is pivotally mounted at its forward end on the base 41 and is urged in an upward direction by a spring 67. The rear facing end of the latch 66 has a surface 68 inclined rearwardly and downwardly. This surface is arranged to engage a transverse bar 69 on the carriage 43.

An inking roller driven by an ink motor 37 (see FIGS. 1 and 2) is mounted in the base 41 towards the rear such that this inking roller rotatably engages with the rearmost one of the inking rollers 45 when the carriage 43 is in its start position.

A metal wheel is provided at each end of the print roller 44 rigidly connected thereto and a metal track surface is pro vided on opposite sides of the plate bed 42 such that when the print roller 44 is in its depressed, printing, position the roller 44 is driven across the plate bed 42 by the metal to metal contact of the metal wheel and the metal track. The racks 52 are spring mounted in the vicinity of the metal track so that the racks 52 do not interfere with the metal to metal drive of the print roller 44.

A transverse buffer 70 is provided at the forward end of the plate bed 42 for final retardation of the carriage 43 at its most forward position.

In FIG. 1 there is shown a circuit diagram for the drive and control of a direct current motor with a three-phase electric supply.

A transformer I (useable as an auto-transformer) supplies power to the field circuit II and to the armature circuit III, of the print roller drive motor. The armature circuit III includes a braking control circuit IV and the field circuit II is subject to the control of a delay circuit V.

The voltage taken from the secondary coil of the transformer I can be varied by a speed switch 34 having three positions, high, medium, and low for adjusting the speed of rotation of the electric motor. In this embodiment the three selectable voltages are 140, 160 and 180 volts for the armature circuit III, the voltage applied to the field circuit Il being 230 volts. The three-phase supply is 440 volts.

The delay circuit V includes a forward control relay FCR, a reverse control relay RCR, and a delay control relay DRR, one side of the coil of each of these relays being connected directly to a terminal 9 at one end of the secondary coil of the transformer. The respective other side of the coil of the relay FCR is connected to a terminal 7 at the respective other end of the secondary coil of the transformer through a normally closed set of contacts RC7 of the relay RCR, a push-button start switch FS, a forward stop microswitch FMS which is the microswitch 54in FIG. 3, and a fuse F2. Similarly, the respective other side of the coil of the relay RCR is connected to the terminal 7 through a normally closed set of contacts FC7 of the relay FCR, a push-button reverse switch RS, a reverse stop microswitch RMS, which is the microswitch 55 in FIG. 3, and

the fuse F2.

Reference hereinafter to sets of contacts preceded by the letters RC, FC, DR, and BC, refer to contacts of the relays FCR, RCR, DRR, and a brake control relay BCR respectively.

The relay BCR is connected on one side to the terminal 9 and on the other side to the terminal 7 through self-holding contacts BCl, two safety microswitches SMSl and SMS2 which are the microswitches 58 in FIG. 1, a push-button stop switch ES normally open contacts DR2, and the fuse F2. Normally open contacts FCl and normally open contacts RC1 are each arranged in parallel with the contacts BCl.

Normally open contacts BC3, normally open contacts FC2, and an inch control switch ISl, all connected in series, are connected in parallel with the switch FS. Similarly, normally open contacts BC2, an inch control switch [S2, normally open contacts RC2, all connected in series, are connected in parallel with the switch RS. Normally closed contacts DR3 and a cycle control switch CS arranged in series are connected between a normally open contact of the microswitch FMS and the wire connecting the switch 182 and the contacts RC2.

The respective other side of the coil of the relay DRR is connected to the speed switch 34 through a resistance R4 of 10 kilohms, a diode D1, normally open contacts RC5 and FC5 arranged in parallel, and a fuse F1. A capacitor C5 of 200 microfarads and normally open contacts DRl connected together in series are connected in parallel with the coil of the relay DRR. A capacitor C6 of 4 microfarads and a resistance R5 of 100 ohms connected together in series are connected in parallel with the coil of the relay DRR and the resistance R4.

A terminal a of a rectifier REC2 for the field circuit II is connected to the terminal 9 and a terminal b of the rectifier REC2 is connected to the terminal 7 through the contacts DR2 and fuse F2. The negative side of therectifier REC2 is connected to one side of the field coil 35 of the print roller motor and the positive side of the rectifier REC2 is connected to the other side of the field'coil 35 through a fuse F4. A capacitor C4 of 47 microfarads and a resistance R2 of 3.3 kilohms connected together in series are connected in parallel with the field coil 35, and a resistance R3 of kilohms is connected in parallel with the capacitor C4. A capacitor C2 of 0.47 microfarads is connected between the terminals a and b of the rectifier REC2.

A terminal a of a rectifier RECl for the armature circuit III is connected to the terminal 9 of the transformer. A terminal b of the rectifier RECl is connected to the speed switch 34 through normally open contacts BC4, the parallel contacts RC5 and FCS, and the fuse F 1. The negative side of the rectifier RECl is connected to a terminal 28 of the armature 36 of the print roller motor through normally open contacts FC3, and the positive side of the rectifier REC] is connected to a terminal 27 of the armature 36 through a fuse F3 and normally open contacts FC4. A resistance R1 of 10 ohms and a capaci tor C3 of 0.47 microfarads connected together in series are connected in parallel with the contacts FC4, the armature 36, and the contacts FC3. Normally open contacts RC4 are connected in parallel with the armature 36 and the contacts F C3. Normally open contacts RC3 are connected in parallel with contacts F C4 and the armature 36.

Normally closed contacts FC6, normally closed contacts RC6, and a potentiometer VRl of 250 ohms, all connected together in series, are connected in parallel with the armature 36. Normally closed contacts BC5 are connected in parallel with the potentiometer VRl. A capacitor C1 of 0.47 microfarads is connected across the terminals a, b of the rectifier RECl.

Various numbered terminals are shown in the diagram indicating the terminals which are connected up manually in the circuit, the numbers for these terminals going from 1 to 23 and 26 to 30. The various contacts of the relays are interelated so that if a relay is operated out of sequence or if one or more of the contacts fail to operate, the operation of the machine will either stop or the incorrect opening or closing of contacts will not effect the operation.

The operation of the proof-printing machine of the embodiment is as follows:

The print roller 44 is driven in forward rolling motion by the drive motor upon pressing switch FS which actuates the relays FCR and DRR.

The closure of the contacts FCl actuates the relay BCR which closes the contacts BCl and the closure of the contacts DR2 holds current supply through contacts BC] to relay BCR. Contacts FC2 and BC3 close to hold current supply through microswitch FMS and contacts RC7 to the relays FCR and DRR when start button FS is released. Contacts FC3, FC4, FCS and BC4 close to supply to the armature and contacts FC6 and BCS open to inhibit the passage of current through the potentiometer VRl or the by-pass including contacts BCS. Current to the motor field coil is fed through closed contacts DR2. The motor thus drives the print roller 44 forward until it trips microswitch FMS.

At the start position of the carriage 43 the force due to the drive of the print roller motor is sufiicient to overcome the force due to the latch spring 67 and the reaction produced thereby from the surface 68, and therefore the transverse bar 69 rides over the latch 66. When the actuator roller 61 meets the first cam switch 62, the actuator arm 60 is displaced upward and therefore the print roller 44 is brought down to the plate bed 42 so that a sheet held in the clips 48 has the required image printed on to it. When the roller 61 reaches the second cam switch 63, the cam lever is pivoted out of the path of the roller 61 and pivots back after the passage of the roller 61.

The tripping of microswitch FMS cuts the supply of current to relays FCR and DRR. Contacts FCl-FCS immediately open and contacts FC6 immediately close. Contacts DRl and DR2, however, are maintained closed for a delay period of approximately one half-cycle of the motor by the capacitor C5. During the delay, therefore, the relay BCR remains energized through contacts DR2 and BCl, holding contacts BC3 and BC4 also closed and contacts BC5 open. Also, current supply is maintained to the field winding of the motor through the closed contacts DR2 for the duration of the delay.

Upon the opening of contacts FC3 and FC4, the motor becomes a generator as it tends to rotate under its own momentum and that of the print roller 44 and the carriage 43. The voltage thus generated is dissipated, via contacts FC6 (now closed) and RC6, through the potentiometer VRl at a rate according to the rotational speed of the print roller 44, and hence the armature 36, and the setting of the potentiometer VRI. The greater the speed of the roller the greater will be the voltage generated and proportionately greater will be the braking reaction between the field and armature windings. The rate at which the generated voltage is dissipated through the potentiometer VRl depends on the setting of the latter and governs the amount of braking force applied to the print roller. It is important to note that there is a constant voltage applied to the field coil windings at all times by means of the auto-transformer (circuit I). Roller speed variations may be obtained through different voltages in the armature as adjusted by the speed switch 34. It is also important to note that the half-cycle delay maintains the field windings energized until the braking is completed; subsequently the opening of contacts DR2, upon de-energization of relay DRR, cuts off the current to relay BCR causing contacts BC5 to close. Any remaining generated voltage charges through contacts BC5 (now closed) and holds the roller armature to a dead stop. In an emergency, the roller can be brought to a dead stop by pressing the emergency button ES which cuts of current supply to relay BCR to effect immediate closure of contacts BC5. The'safety microswitches SMSl and SMS2 effect the same emergency stop when actuated. In practice the braking of the motor reduces the speed of rotation of the print roller 44 before the carriage 43 meets the buffer 70. Therefore, when the carriage 43 meets the buffer 70 there is very little momentum for the buffer 70 to absorb. The machine is so arranged that, for any possible speed of the print roller 44 and therefore the carriage 43, the carriage 43 meets the buffer 70 before the discharge of the capacitor C5 de-energizes the relay DRR, and the contacts BC5 are closed when the print roller 44 and therefore the armature 36 are already stationary. Braking of the print roller 44 by opening the contacts BC5 jars the machine and therefore is to be avoided except in an emergency when braking of the roller in the shortest possible time and distance is expedient.

At the completion of the forward stroke of the roller 44 and when the cycle switch CS is on" i.e., closed, the roller 44 will commence its reverse stroke. The microswitch FMS is a changeover switch and as soon as it is tripped and during the halfcycle delay period, it passes through the contacts DR3, BC2 and FC7 (all closed) to the relay RCR which is energized to actuate contacts RC1 to RC7 in the same way as the corresponding alternative contacts FCl to FC7 are actuated in the forward stroke as already described. The sequence of operation is equivalent to that of the forward stroke and the tripping of the microswitch RMS effects the same braking characteristic.

When, however, the cycle switch CS is off, i.e., open, it is necessary to press the button switch RS to initiate the reverse stroke of the print roller. This allows for manual control of the proof-printing machinery. An additional feature resides in the provision of the inch switch composed of paired contacts 181 and 152 the opening of which allows forward and reverse stepping of the print roller 44. When stepping, the braking feature comes into operation as if the roller were completing a full stroke. For stepping the print roller 44, the paired inch switch contacts 181 and 152 are opened and the motor is rotated in either the forward or reverse direction by pressing the respective button FS or RS, release of the pressed button causing the braking effect, using thepotentiometer VRl, to

commence.

When the carriage 43 is reversing, the actuator roller 61 is displaced by the second cam switch 63 causing the actuator arm 60 to be lowered and therefore the print roller 44 to be raised from the bed 42, so that on the return stroke the print roller 44 does not cause smudging of the already printed sheet by contact with the printing plates on the bed 42. When the roller 61 reaches the cam switch 62, the cam lever is rotated out of the rollers path and rotates back after the roller 61 has passed through.

As for the forward motion, the reverse motion is braked proportionately to the speed of the carriage and roller, so that the carriage 43 is moving relatively slowly when it reaches the buffer springs 65. The momentum of the carriage 43 is sufficient to depress the latch 66 so that the transverse bar 69 rides over the latch. When buffer plates on the carriage 43 strike the buffer springs 65, the springs are compressed to bring the carriage 43 to a halt and then extend slightly returning the carriage towards the latch 66. The return motion of the carriage 43 is not sufficient to over-ride the latch 66 and therefore the carriage comes to a halt held in equilibrium by the buffer springs 65 on the hand and the latch springs 67 on the other hand. This mechanical arrangement for holding the carriage 43 in its start position obviates the need for a locking latch which would need to be unlocked before the carriage 43 could commence its forward movement. The braking of the carriage 43 by the print roller motor acting as a generator ensures that the carriage 43 is only moving relatively slowly when it reaches the buffer springs 65 and therefore the retardation of the carriage 43 by the buffer springs 65 does not cause harmful jar to the machine. Since the braking by the print roller motor is proportional to the speed of rotation of the print roller 44, the faster that the print roller 44 rotates causes a proportionately greater braking force, and therefore the motion of the carriage 43 is always braked sufficiently before the carriage 43 reaches the buffer springs 65. The microswitch RMS, i.e., the microswitch 55, is attached to the base 41 at a sufficient distance from the rear of the base 41, so that for any possible speed of the carriage 43 in the reverse direction, there is sufficient distance for the speed to be reduced before the carriage reaches the buffer springs 65. Also, due to the proportionate braking of the print roller 44, it is not possible for the carriage 43 to be braked to such an extent that it does not reach the buffer springs 65.

The circuit shown in FIG. 2 is the transformer arrangement used when a single phase supply drives the machine. The terminals 1 to 9 shown in FIG. 2 are connected as the terminals 1 to 9 shown in FIG. 1 in place of the transformer arrangement for the three phase supply.

Modifications are possible. For example, instead of the delay arrangement, shown in FIG. 1, including the relay DRR and the capacitor C5, suitable high voltage capacitors may be connected in parallel with the field coil 35 to achieve the necessary delay and to cause the motor to act as a generator by direct current injection.

A direct current motor or an alternating current motor may be used. A transformer is not essential, however, if omitted buffer resistances will be needed across the armature from surge currents. Other forms of switching circuits may be used, for example, the relays may be replaced by relayless solid state switching circuits.

The reverse drive need not be provided, for example, the racks 52 may be replaced by an endless chain, so that for reverse drive the print roller 44 is merely driven by a sprocket around the turn of the endless chain and back along to the start position, the drive motor rotating all the time in a forward direction.

For the braking operation, the power may be maintained in the armature circuit, for example when using a squirrel cage motor, so that the current is generated from the field circuit which would then incorporate the potentiometer. The capacitors C1 and C2 may be dispensed with if the other components were selected such that the smoothing due to these capacitors was not needed.

The drive apparatus is not limited to proof-printing machines but may be used, for example, for a rotary press offset printing machine having a reciprocating action of either the bed of the carriage.

A plurality of plate beds may be connected together in line, a single start position for the carriage being provided at the rear end of the linked plate beds and a forward stop microswitch provided at the forward end of each bed so that the plate on the first bed can be proof-printed and adjusted as necessary, then the plates on the first and second beds can be proof-printed, and so on until all of the plates are proofprinted at one time.

What I claim is:

1. In a printing machine comprising a plate bed and a printing roller mounted for rotational movement along the plate bed, means for controlling the rotational movement of the roller, said means comprising a shunt-wound motor having a field winding and an armature winding, means coupling the shunt-wound motor with the printing roller for effecting rotation of the printing roller, electrical circuit means for electrically connecting a power source to the field winding and the armature winding and for controlling the speed of rotation of the motor and thus of the printing roller, the electrical circuit means including switch means actuable to cause a braking 3O force to be applied to the motor at the end of a printing operation, the braking force being proportional to the speed of the motor and thus of the printing roller, said switch means being actuable to isolate the armature winding of the motor from the power source at the end of a printing operation while the motor and the printing roller are efiecting said rotational movement to cause the motor to function as a generator to produce said braking force, and the circuit means including first relay means for connecting the windings of the motor to said power source to effect rotation of the motor in a forward direction, second relay means for connecting the windings of the motor to said power source to effect rotation of the motor in a reverse direction, third relay means controllable by the first and second relay means individually for connecting the circuit means to cause electric current to flow in the field windings of the motor, and delay circuit means for temporarily maintaining the third relay means in its actuated state when the respective one of the first or second relay means has ceased to actuate.

2. Means as claimed in claim 1, wherein the circuit means includes a variable resistor connected in parallel with the armature winding of the motor, a predetermined value of resistance of the variable resistor determining the proportionality of the motor braking force with respect to the speed of rotation of the motor.

3. Means as claimed in claim 2, wherein the circuit means includes means for short-circuiting the variable resistance when no electric current flows in the field winding. 

1. In a printing machine comprising a plate bed and a printing roller mounted for rotational movement along the plate bed, means for controlling the rotational movement of the roller, said means comprising a shunt-wound motor having a field winding and an armature winding, means coupling the shunt-wound motor with the printing roller for effecting rotation of the printing roller, electrical circuit means for electrically connecting a power source to the field winding and the armature winding aNd for controlling the speed of rotation of the motor and thus of the printing roller, the electrical circuit means including switch means actuable to cause a braking force to be applied to the motor at the end of a printing operation, the braking force being proportional to the speed of the motor and thus of the printing roller, said switch means being actuable to isolate the armature winding of the motor from the power source at the end of a printing operation while the motor and the printing roller are effecting said rotational movement to cause the motor to function as a generator to produce said braking force, and the circuit means including first relay means for connecting the windings of the motor to said power source to effect rotation of the motor in a forward direction, second relay means for connecting the windings of the motor to said power source to effect rotation of the motor in a reverse direction, third relay means controllable by the first and second relay means individually for connecting the circuit means to cause electric current to flow in the field windings of the motor, and delay circuit means for temporarily maintaining the third relay means in its actuated state when the respective one of the first or second relay means has ceased to actuate.
 2. Means as claimed in claim 1, wherein the circuit means includes a variable resistor connected in parallel with the armature winding of the motor, a predetermined value of resistance of the variable resistor determining the proportionality of the motor braking force with respect to the speed of rotation of the motor.
 3. Means as claimed in claim 2, wherein the circuit means includes means for short-circuiting the variable resistance when no electric current flows in the field winding. 